I was having a look again at Round 13 "Messed Up Link" from last year's 2012 WPC in Croatia.
The first puzzle is called ABCD Neighbours, and has no instructions.
At the time I couldn't deduce the rules. And now I still can't figure them out.
I recall they were a little complicated, and I think the WPC organisers printed them out and pinned them to a noticeboard, but I can't find them in the Solutions pdfs or online anywhere.

Wasn't it something simple, like this?
"A pair of letters means those letters must be adjacent to each other in the order given. The pair can have no more than 1 other neighbour" [Edit: col5 of example contradicts this bit]
"A single letter can have no more than 1 neighbour"
"Every letter appears once in each row/col"

I'm looking at the puzzle, having taken the WXYZ from the solution, to see if the above gives a unique answer.

Thanks Alan, it's just the meaning of the single letter that I was after, and this rule looks to always be true. I was getting distracted that they were usually at an end of the row/column. I'll test it out.

James - I have vague memories of Nikola and the Serbians solving it up on that noticeboard in Croatia in case you needed any further reference. My personal memories of it were being utterly baffled during competition and being (almost!) through with wanting to solve puzzles afterwards!

OK, so that rule wasn't enough, it gives multiple answers for the example. I think it's actually:

- A single letter cannot be adjacent to the given pair of that row/column.
- And if no pair is given, then the single letter cannot be adjacent to any pair of letters (although this only occurs in one of the 4 puzzles, so not a great idea in an instructionless puzzle)

Tom, it was a baffling round, I was just glad it was the last one, exhausted after 12 rounds of puzzles and ready for dinner.
Looking at it again now, I can see that there were 5 answer sets that repeated cyclically through the 20 puzzles. But the puzzles are hard enough even if you know exactly what's going on.